Interferon was found to be mainly or solely responsible for augmentation of mouse and human natural killer (NK) cell activity. This effect was dependent on transient new RNA and protein synthesis. NK boosting agents could also induce syppression of activity. Human NK cells appear to be the same as effector cells of antibody-dependent cellular cytotoxicity. NK cells appear to have a role in in vivo anti-tumor resistance. Cytotoxicity by human monocytes and granulocytes was boosted by interferon and other lymphokines. Depressed mixed leukocyte culture responses in lung cancer patients was predictive of disease recurrence. Immunotherapy of cancer patients with BCG or Corynebacterium parvum was shown to cause rapid changes in cytotoxicity by NK cells and monocytes.